Radio and RADAR circuits and systems operating at high frequencies (e.g. microwave frequencies), are increasingly being implemented as integrated structures. Such systems typically include a scheme to couple microwave electromagnetic energy to and/or from the integrated structure. For example, off-chip antennas have been used with wirebond or flip-chip technologies. FIG. 1 shows the use of wire-bond technology to couple electromagnetic energy from chip 111 to an off-chip antenna 102 disposed on a printed circuit board (PCB) 101 via a transmission line 110. Such wire-bond technologies have been extensively used to build relatively low-frequency connections (below 5 GHz). The typical diameter of the gold wires used in such wire-bond technology is about 25 μm. These stand-alone wires provide a parasitic inductance of about 1 nH/mm which introduces a large mismatch in mm-wave frequencies. To resonate out the inductance of these wires and carry a high frequency signal, a ground wire should be placed adjacent to the signal wire with the right distance to maintain a good impedance match (usually 50Ω) to the antenna. Placing such a wire requires an accurate control of the wirebond profile (to an accuracy of about 1 μm) which significantly increases the packaging cost in high-volume production.
In applications requiring a smaller parasitic inductance, flip-chip technology is typically utilized. In flip-chip technology, as shown in FIG. 2, the silicon die is flipped on top of a PCB (e.g. flip chip 210) and gold or solder bumps 211 are used to connect the on-chip pads to PCB traces leading to the PCB transmission line 110. Today's flip-chip technology provides gold bumps as small as 50-100 μm in diameter and height and introduces a relatively small parasitic inductance compared to wire-bond technology. However, the shape of the bump is not predictable enough (accuracy of 1 μm) to be used in relatively high frequency applications (higher than 50 GHz).
One of the important disadvantages of conventional silicon-based on-chip antennas is the low antenna efficiency. The low antenna efficiency is a result of two factors; silicon's high dielectric constant (11.7), and the substrate's low resistivity (1-10 Ω·cm). The high level of doping required to fabricate active circuits limits the silicon substrate's resistivity. Also, as previously reported by the present inventors in Babakhani, et al., A 77 GHz phased array transceiver with on chip dipole antennas: Receiver and on-chip antennas, IEEE Journal of Solid-State Circuits, vol. 41, no.12, pgs. 2795-2806, December 2006, and Babakhani, et al., A Near-Field Modulation Technique Using Antenna Reflector Switching,” in IEEE Antennas and Propagation International Symposium, June 2007, pgs. 4369-4372, and shown in the illustration of FIG. 3, both the high dielectric constant of silicon and a relatively large substrate thickness (200-300 μm) cause most of the on-chip antenna output power to be coupled into substrate-modes in unshielded structures.
Use of an on-chip ground shield to isolate the on-chip antenna from the lossy substrate causes the radiation efficiency to be very small (around 1%). In standard silicon processes the distance between on-chip metal layers rarely exceeds 15 μm. A ground layer at this distance, which is much smaller than the wave-length in mm-wave frequencies (e.g. 2.5 mm wavelength in SiO2 at 60 GHz), shorts the antenna by introducing a negative image current very close to the antenna and hence reduces both the radiation resistance and the efficiency. On the other hand, if an on-chip ground shield is not used, the silicon substrate behaves as a dielectric waveguide, generates the substrate modes, and leads the power to the chip edges resulting in an undesirable pattern. Thus, due to the silicon substrate's low resistivity most of the power that couples into substrate-modes disappears as heat reducing the overall antenna efficiency.
Among the conventional ways to alleviate the substrate coupling problem, a silicon lens can be also used at the backside of the chip such as has been described by Babakhani, et al, in A 77 GHz phased array transceiver with on chip dipole antennas: Receiver and on-chip antennas, IEEE Journal of Solid-State Circuits, v.41, no. 1 2, pgs. 2795-2806, December 2006, Babakhani, et al, in A Near-Field Modulation Technique Using Antenna Reflector Switching, in IEEE Antennas and Propagation International Symposium, June 2007, pgs. 4369-4372, and Rutledge, et al., in Integrated-circuit antennas, Infrared and Millimeter-Waves. New York: Academic, 1983, pgs. 1-90. By attaching an un-doped silicon hemispherical lens (or a dielectric lens with a dielectric constant similar to that of silicon) to the backside of the substrate as shown in FIG. 4, antenna efficiencies of about 10% can be achieved. Due to the impedance mismatch between the silicon and the air, about 30% of the radiated power is reflected from the silicon-air boundary
      (                  Z        si            =                                                  μ              0                                      ɛ              si                                      =                              110            ⁢            Ω            ⁢                                                  ⁢            versus            ⁢                                                  ⁢                          Z              air                                =                                                                      μ                  0                                                  ɛ                  0                                                      =                          377              ⁢                                                          ⁢              Ω                                            )    .Because the wave travels inside the silicon lens before reaching to the air, it gets attenuated due to the non-idealities and doping of the silicon lens. While use of a silicon lens substantially increases the complexity of the package and its manufacturing cost, it still remains as one of the effective methods in implementing on-chip antennas.
What is needed, therefore, is a more efficient and cost effective way to couple relatively high frequency signals to an off-chip antenna.